A sulfuric acid hydrolysis process in which acrylamide sulfate is obtained by heating acrylonitrile in the presence of sulfuric acid and water was used initially as an industrial process for the production of acrylamide. This process, however, has been replaced by a copper, catalyst process in which acrylamide is obtained by direct hydration of acrylonitrile in the presence of a copper catalyst such as copper metal, reduced copper, Raney copper, or the like. In addition, a microbiological method in which a microbial nitrile-hydration enzyme (nitrile hydratase) is used has recently been developed and put into practical use as an industrial process for the production of high purity acrylamide.
Of these acrylamide production processes, the copper catalyst process is apt to cause side reactions because of the generally high reaction temperature (60.degree. to 150.degree. C.) and reaction pressure (0 to 20 kg/cm.sup.2), thus requiring a refining step for the removal of by-products and other incidental impurities such as catalyst-originated metal ions. In the case of the microbiological method, on the other hand, there are no impurities such as metal ions as a matter of course, and the amount of by-products is markedly small in comparison with the copper catalyst process, because the enzyme reaction is effected under ordinary temperature and pressure, thus rendering possible simplification of a refining step or even its omission. However, when a high performance polymer is produced for use in the aforementioned coagulating agent and the like, it is necessary to increase the purity of acrylamide as much as possible.
However, as in the case of many other unsaturated monomers, acrylamide is apt to cause polymerization not only by its exposure to light or heat but also by its contact with an iron surface, and such properties cannot be altered by improving purity of its aqueous solution.
Because of its unstable nature, acrylamide is usually kept at a low temperature (about 20.degree. C.) in the dark, avoiding contact with an iron surface, in addition to the use of a stabilizing agent therewith.
A number of stabilizing agents have been proposed for this purpose, including, for example: 8-hydroxyquinoline and cupferron iron salt (JP-B-39-23548); thiourea, ammonium thiocyanate and nitrobenzene (JP-B-39-10109); ferron (JP-B-40-7171); furildioxime (JP-B-40-7172); a chrome-cyanogen complex (JP-B-41-1773); p-nitrosodiphenylhydroxylamine (JP-B-45-11284); 2,6-di-t-butyl-3-dimethylamino-4-methylphenol (JP-B-47-4043); 4-aminoantipyrine, oxalic acid and hydroxylamine sulfate (JP-B-47-28766); and a mixture of manganese with a chelate compound (JP-B-48-3818). (The term "JP-B" as used herein means an "examined Japanese patent publication")
These stabilizing agents are used in the acrylamide production process for the purpose of preventing polymerization, stabilizing precipitated crystals, and stabilizing an aqueous acrylamide solution. These agents are classified as polymerization inhibitors or polymerization retarders. As a consequence, when compounds among these stabilizing agents have a low polymerization retarding ability, they must be used in a considerably large amount, which is not economical, while those having a high retarding capacity will exert an adverse influence on polymerization, even in a small amount. In addition, these stabilizing agents are not always effective when used to stabilize an aqueous acrylamide solution which is in contact with an iron surface. In actuality, it is almost impossible to protect acrylamide perfectly from contact with an iron surface during its production, purification and storage steps, for example, from its local contact with an iron surface caused by a pin hole or peeling of a vessel lining, exposure of a weld zone of piping, or the like.